There have been developed, e.g., Digital Audio Tape Players (hereinafter simply referred to as DAT) adapted for recording and/or reproducing digital audio data with respect to magnetic tape and Digital Data Storages in which such DAT is used to record or reproduce data of the computer. In these devices, magnetic tape is traveled in the state where the magnetic tape is wound at a wrap angle of, e.g., 90 degrees on the rotary drum, and the rotary drum is rotated to carry out recording/reproduction in accordance with the helical scan system by using the magnetic head on the rotary drum. Thus, high density recording is permitted.
In accordance with this helical scan system, inclined tracks TK.sub.A, TK.sub.B are formed on the magnetic tape as shown in FIG. 1. These inclined tracks TK.sub.A, TK.sub.B are tracks formed by, e.g., recording in which no guard band is provided (so-called azimuth full recording) by using a pair of recording heads HR.sub.A, HR.sub.B in which azimuth angles are different from each other mounted on the rotary drum. In more practical sense, respective gaps GP of the recording heads HR.sub.A, HR.sub.B are adapted so that azimuth angles .theta..sub.A, .theta..sub.B are provided in directions opposite to each other, and magnetization (magnetic polarization) directions of the inclined tracks TK.sub.A, TK.sub.B are different from each other as indicated by slanting lines. In this azimuth full recording, the track width TP is narrower than the width of the recording heads HR.sub.A, HR.sub.B.
Meanwhile, at the time of reproduction, the magnetic head is required to precisely trace tracks TK (inclined tracks TK.sub.A, TK.sub.B) on the magnetic tape. As this tracking control system, e.g., in the digital data storage, a tracking servo control so called timing ATF (Automatic Track Following) system is used.
In accordance with this timing ATF system, the time (or time period which will be referred to as tracking detection time) required from the reference phase position of the rotary drum until the magnetic head detects (reproduces) a predetermined signal (hereinafter referred to as a timing detection signal) from corresponding track is measured to compare that measured value and reference value to allow error therebetween to be servo error information. Then, the rotational velocity (speed) of the capstan motor for traveling the magnetic tape is controlled on the basis of this servo error information to adjust the tape traveling velocity. Namely, the tape traveling velocity is adjusted to adjust the relative velocity between the drum rotational velocity and the tape traveling velocity so that a satisfactory tracking state can be obtained.
In more practical sense, as shown in FIG. 3, phase position of the rotary drum when scanning position of the magnetic head is caused to be the position of line (timing) designated as TR.sub.A in FIG. 3 with respect to a certain track is caused to be the reference phase position. At the rotary drum or the drum motor, pulse generator (PG) is disposed. At the time point when the rotary drum is located at the reference phase position during rotation, a pulse signal is generated from the PG. Accordingly, it is possible to detect the timing TR.sub.A at which the rotary drum has been located at the reference phase position. Thereafter, when the magnetic head comes into contact with the magnetic tape to scan the inclined track TR.sub.A, a timing detection signal is detected as reproduction data at a predetermined position P.sub.TTP on the track. This timing detection signal serves so that pulse can be obtained at the predetermined position P.sub.TTP such as synchronizing signal or address, etc. included in reproduction data.
In this case, three different kinds of scanning loci in the tracking phase state of the magnetic head with respect to the inclined track TK.sub.A are designated as 1, 2, 3 in FIG. 3. The times from the timing when the magnetic head is located at the reference phase position (position of the line TR.sub.A) of the rotary drum up to the timing when the magnetic head reaches the position P.sub.TTP, i.e., tracking detection times are different from each other as respectively designated as t1, t2, t3 in the scanning states of 1, 2, 3.
As the reference value of the tracking detection time, time t1 obtained when the magnetic head is placed in the satisfactory tracking state with respect to the track TK, i.e., in the state where it traces the center of the inclined track TK.sub.A as in the case of 1 is set in advance. Accordingly, in the case where scanning as in the case of 1 is carried out at the time of tracking servo control so that time t1 is measured as the tracking detection time, the measured value becomes in correspondence with the reference value. Namely, in this case, there is no error between the measured value and the reference value, and there results satisfactory tracking state. On the other hand, in the case where scanning is carried out in the tracking phase state as in the case of 2 or 3, the measured value of the tracking detection time becomes equal to t2 or t3, giving rise to error with respect to the reference value. In this case, there results the state where tracking deviates by that error. By reflecting this with respect to the tape traveling velocity, the servo control then results in just the tracking state can being carried out.
In providing (applying) tracking servo (control) by such timing ATF system, it is necessary to determine in advance the reference value. This reference value is the time (time period) from the timing of the reference phase position of the rotary drum in the just tracking state to the timing at which the timing detection signal is detected. Since the timing detection signal is generated, e.g., on the basis of detection of synchronizing signal, etc. at a predetermined position on the track, that position P.sub.TTP should be fixed at each track of various tapes. However, from a practical point of view, it cannot be avoided that positional shift may take place resulting from mechanical error, etc. at various recording equipments and reproducing equipments. For this reason, in the digital data storage, in the case where certain file data is reproduced, it is necessary to measure, prior to read-out of that data, the reference value at that tape (its file data track).
In this measurement of the reference value, there is conducted such a processing to carry out scanning in various tracking phase states with respect to the tracks to calculate, from tracking detection times measured at respective scanning operations, e.g., an average value of these detection times to allow such an average value to be the reference value.
Its operation image is shown in FIG. 4. As shown in FIG. 4, when scanning is carried out in plural tracking phase states TJ1.about.TJ5 different from each other with respect to, e.g., the inclined track TK.sub.A to calculate an average value of respective tracking detection times measured in those scanning operations, tracking detection time in the tracking phase state in the vicinity of the tracking phase state TJ3 is obtained. This tracking detection time is permitted to be tracking detection time substantially in the just tracking state. Therefore, it is sufficient to employ this tracking detection time as the reference value.
Meanwhile, in the digital data storage etc., reproduction must be carried out with good compatibility with respect to tape cassettes recorded by various recording devices. When this applies to tracking, satisfactory tracking servo controls by the timing ATF system must be carried out with respect to various tape cassettes.
When ideally viewed, the relationship between the track TK on the magnetic tape and scanning locus of the reproduction head is such that, as shown in FIG. 5(a), track TK is linearly formed and scanning locus of the magnetic head at the time of reproduction is in a linear form. In such an ideal state, when an approach is employed to compare tracking detection time based on timing detection signal obtained at a predetermined position P.sub.TTP on the track and reference value of tracking detection time set in advance by the operation for setting reference value to carry out servo control so that its error becomes equal to zero, scanning can be carried out in the satisfactory tracking state from the beginning to the end of the track.
However, between various recording devices, there are mechanical errors, such as, for example, mounting position, etc. of the recording head with respect to the rotary head, and the tape traveling velocity at the time of recording is not strictly constant (fixed), but changes to some extent. Further, also in reproducing devices, there are mechanical errors such as mounting position, etc. of the reproducing head, and the tape traveling velocity also changes. It cannot be said from these various factors that ideal tracking state as shown in FIG. 5(a) can be always obtained at the time of reproduction.
For example, as shown in FIG. 5(b), there are instances where the track TK is formed in a curved manner at the time of recording. Contrary to this, scanning locus of the reproduction head of the reproducing device is assumed to be substantially in a linear form. In this case, when setting of reference value and servo control at the time of reproduction are carried out by timing detection signal obtained at a predetermined position P.sub.TTP on the track, a tracking servo control such that just tracking state can be obtained, e.g., at the position P.sub.TTP is carried out. Namely, the scanning locus of the reproduction head results in a locus as indicated by single dotted lines in the figure. In this case, since the track TK is curved as shown in FIG. 5(b), scanning of the reproduction head deviates from the track at the last (end) portion of the track TK. In other words, satisfactory tracking state fails to be maintained over the entirety of the track TK.
Moreover, in a device such that the track TK is formed in a linear form, but the scanning locus of the reproduction head is curved as shown in FIG. 5(c), for example, when setting of the reference value and servo control at the time of reproduction are similarly carried out by the timing detection signal obtained at the position P.sub.TTP, the scanning locus of the reproduction head results in a locus as indicated by a single dotted line in FIG. 5(c), thus failing to maintain satisfactory tracking state over the entirety of the track TK.
Namely, in circumstances as shown in FIGS. 5(b), 5(c), for example, even if the tracking servo normally functions, satisfactory tracking is not carried out in practice. As a result, the error rate of reproduction data is deteriorated, leading to lowering of reliability of the device.
Meanwhile, if servo (control) is provided (applied) so that scanning locus as indicated by solid line in FIGS. 5(b), 5(c) can be obtained, tracking state of the range allowable over substantially the entirety of the track can be realized. Thus, data recorded on the track can be read out without problem. In view of this, in order to permit realization of such a scanning, there is conceivably a method in which the track is divided into plural recording areas so that timing detection signals can be obtained at plural positions to carry out the setting of the reference value on the basis of these timing detection signals.
For example, as shown in FIG. 6, four recording areas R1.about.R4 are provided at the track TK to set positions P.sub.TTP 1.about.P.sub.TTP 4 where timing detection signals can be obtained within respective recording areas R1.about.R4. For example, setting is made such that timing detection signals can be obtained by addresses or synchronizing signals at four portions included in recorded data on the track. Then, tracking detection times corresponding to distances from the reference phase position TR of the rotary drum to respective positions P.sub.TTP 1.about.P.sub.TTP 4 are measured.
In more practical sense, with respect to the position P.sub.TTP 1 time tR1 from the time point of the reference phase position TR to the time point when timing detection signal can be obtained at the position P.sub.TTP 1 is measured to allow this time tR1 to be tracking detection time. With respect to the position P.sub.TTP 2, time tR2 from the time point of the reference phase position TR to the time point when the timing detection signal can be obtained at position P.sub.TTP 2 is measured to allow the value obtained by subtracting the standard time difference TLa from the above-mentioned time tR2 to be tracking detection time. This standard time difference TLa is the standard time required for scanning from the position P.sub.TTP 1 to the position P.sub.TTP 2. With respect to the position P.sub.TTP 3, time tR3 from the time point of the reference phase position TR to the time point when the timing detection signal is obtained at position P.sub.TTP 3 is measured to allow value obtained by subtracting the standard time difference TLb from the above-mentioned time tR3 to be tracking detection time. This standard time difference TLb is the standard time required for scanning from the position P.sub.TTP 1 to the position P.sub.TTP 3. With respect to the position P.sub.TTP 4, time tR4 from the time point of the reference phase position TR to the time point when the timing detection signal is obtained at position P.sub.TTP 4 is measured to allow value obtained by subtracting the standard time difference TLc from the above-mentioned time tR4 to be tracking detection time. This standard time difference TLc is the standard time required for scanning from the position P.sub.TTP 1 to the position P.sub.TTP 4.
When tracking detection times corresponding to respective positions P.sub.TTP 1 to P.sub.TTP 4 are measured as stated above in an ideal state as shown in FIG. 5(a), tracking detection times corresponding to the respective positions P.sub.TTP 1.about.P.sub.TTP 4 take the same value. However, since ideal tracking states are respectively different in the respective recording areas R1.about.R4 under the state as shown in FIGS. 5(b), 5(c), tracking detection times in the just tracking state at respective positions P.sub.TTP 1.about.P.sub.TTP 4 take different values.
In this case, when an average value of respective tracking detection times corresponding to the respective positions P.sub.TTP 1.about.P.sub.TTP 4 is determined, this average value takes a value capable of obtaining equivalent tracking state to some extent in the respective recording areas R1.about.R4. In view of the above, the operation to determine respective tracking detection times in plural recording areas within one track as stated above is executed in various tracking phase states as shown in FIG. 4 to obtain plural tracking detection times. Then, an average value of these tracking detection times is determined. This average value takes a value corresponding to the tracking state within a range allowable (tolerable) to some extent in the respective recording areas R1.about.R4. Namely, if this average value is assumed as the reference value, scanning as indicated by solid line in FIGS. 5(b), 5(c), for example, can be carried out. Namely, reasonable reproduction operation can be carried out.
However, with such a method, when there is dispersion in the number of tracking detection times measured every recording areas, reference value for obtaining substantially satisfactory tracking over the entire area of the track cannot be obtained. In more practical sense, consider the case where scanning for setting reference value is carried out with respect to the track TK formed in a curved manner as shown in FIG. 7. As has been explained with reference to FIG. 4, it is preferable to obtain the tracking detection time for setting the reference value in diverse tracking phase states. In view of this, it is now assumed that scanning for measurement of the tracking detection time is carried out in tracking phase states designated at 4, 5, 6 in FIG. 7, for example.
In this case, in the case of the scanning 5, timing detection signals can be obtained from respective positions P.sub.TTP 1.about.P.sub.TTP 4, and tracking detection times respectively corresponding to the positions P.sub.TTP 1.about.P.sub.TTP 4 can be thus obtained. However, in the case of the scanning 4, timing detection signals can be obtained only from the positions P.sub.TTP 1, P.sub.TTP 2. As a result, only two tracking detection times corresponding to the positions P.sub.TTP 1, P.sub.TTP 2 can be obtained. In addition, in the case of the scanning 6, timing detection signal can be obtained only from the position P.sub.TTP 4. As a result, only one tracking detection time corresponding to the position P.sub.TTP 4 can be obtained.
The above-mentioned explanation has been given only for illustrative purpose. In practice, the number of tracking detection times corresponding to respective positions P.sub.TTP 1.about.P.sub.TTP 4 considerably varies by such relative shift between the track and the scanning loci. For this reason, the above-described average value does not necessarily result in the value equally reflecting the tracking states with respect to all the recording areas R1.about.R4. As a result, it cannot be said that scanning as indicated by solid line of FIGS. 5(b), 5(c) can be necessarily realized.
Moreover, it is conceivable to employ an approach to determine, at every one of the recording areas, average values of tracking detection times respectively obtained at the positions P.sub.TTP 1.about.P.sub.TTP 4 to further average these average values in the entire recording area. When such an approach is employed, influence of variations in the number of tracking detection times at respective positions P.sub.TTP 1.about.P.sub.TTP 4 can be reduced. However, employment of this average value as the reference value allows the recording area where off track quantity is largely apt to take place. It is now assumed that, e.g., average values in the recording areas R1, R2, R3 are substantially equal to each other, and the average value of the recording area R4 has a greatly different value. In this case, when average values of the recording areas R1.about.R4 are averaged to allow such average value to be reference value, that value becomes equal to a value which has greatly reflected off track quantities in the recording areas R1, R2, R3, i.e., there is the possibility that tracking may greatly deviate in the recording area R4.
As stated above, in the conventional tracking control apparatus employing the timing ATF system, in the case where the relationship between scanning locus of the magnetic head at the time of reproduction and the track shape is not ideal, there was the problem that it is difficult to maintain satisfactory tracking over the entire area of the track in a manner corresponding thereto, resulting in partially deteriorated error rate.